The need often arises in radio frequency devices to have an electrical resistor which can attenuate radio frequency energy. Typically, such devices are designed to operate at a given resonant frequency, but other frequencies are often found present in the device also. These other frequencies are unwanted because they interfere with frequency dependent receivers of the RF power or because the useful RF power of the device is reduced. Various arrangements have been proposed to identify and discriminate against unwanted frequencies, as by absorbing or attenuating the identified unwanted frequencies.
High frequency attenuators or loads fall into two groups which differ in the means by which the power is attenuated. The first group utilizes a waveguide operated at a frequency which is below the characteristic cut-off frequency as by the size and shape of the waveguide. The electromagnetic fields excited at one end of the waveguide couple weakly to a receiving element at the other end of the waveguide, the amount of coupling depending on the length and size of the waveguide. This technique is particularly suitable for application in the microwave region, although it has been employed at frequencies as low as a few megacycles per second. One of the most common microwave attenuators of this type is one which utilizes a waveguide beyond cutoff. It is well known that for a given frequency of oscillation, one can reduce the dimensions of either circular or rectangular waveguides to a point where energy of this frequency can no longer be propagated in the waveguide. Below the cutoff frequency, the fields decay exponentially along the waveguide, and the phases change a negligible amount by losses in the walls. It is possible to excite fields in a waveguide beyond cutoff in several ways, and to couple selectively to a mode of transmission whose attenuating characteristics can be calculated.
In a second group of attenuators, radio frequency power is absorbed in poorly conducting materials and transformed into heat, as in conventional resistors which are used at very low frequencies. For laboratory use, many RF attenuators of this type are available which give fixed or variable attenuation of the main power flow, or which permit power sampling for power-monitoring purposes, without reacting perceptably upon the main power flow. For low power ranges, it is usually permissible to insert in the main power path devices having a dielectric base upon which thin coatings of power-absorbing materials such as carbon or Aquadag are applied. One design utilizes a thermosetting plastic, which is cast in a section of the line to be terminated. The plastic material, Durez 7421, is relatively easy material to machine, and a conical matching taper of the correct length can be cut on a lathe. A second type of load or attenuator has been made from a resistive cloth, Uskon. A long trapezoidal piece of the cloth is tightly wrapped around the inner conductor of the coaxial line in such a way that a conical matching taper, followed by a completely filled length of line is formed. This is a less durable load than the one made from Durez, particularly since the cloth has a tendency to fray at the tip of the taper, thereby producing reflections. Loads of this type may be made for a variety of sizes of coaxial lines, but may become objectionably long at wavelengths longer than microwaves. Various polyiron materials have been used effectively in making step terminations in coaxial lines. Stepped cylinders of the polyiron material have been used to form terminations in coaxial lines operated at wavelengths considerably longer than microwaves. This material has the disadvantage, however, in that its high frequency properties vary considerably from batch to batch, and the dimensions often need to be corrected when units are made from a new batch. Also, polyiron is not an easy material to machine. Diamond-dust grinders and Carboloy-tipped drill bits and lathe tools must be used in cutting and otherwise machining the material. Further, a health hazard is presented to the machinist in the form of iron dust formed during the machining operations.
Other low-power terminations include metalized glass arrangements. In one such device, a thin evaporated Nichrome film is sandwiched between a glass plate support and a thin protective magnesium fluoride film. However, the film material is prohibitively expensive and problems are encountered in providing a glass plate of sufficient mechanical strength. Further, such attenuators have to be made objectionably long in order to provide sufficient attenuation to effectively eliminate reflections from the short circuit of a mechanical holder supporting the device. These longer lengths aggravate the stress in the relatively fragile glass plate. Other uses of metalized-glass dissipative elements in waveguide transmission systems include glass plates which carry a metal film only on one side and which are suspended in the waveguide with the film parallel to the electric field lines. In the simplest construction, the glass plate has one or two holes drilled along its centerline and is cemented to one or two metal struts. The struts penetrate the guide wall, preferably at right angles to the electric field lines. The struts can, in turn, be made to move and carry the glass plate across the waveguide, as in the variable waveguide attenuators. Instead of drilling holes, these attenuators may be formed by employing a fine gas flame to burn out or melt the holes. In either event, specially formed eyelets of German-solder material or the like are crimped onto the glass by special equipment and the struts are soldered to the eyelets.
For larger powers, in order to provide for efficient heat transfer to the ambient air, power-absorbing materials in greater bulk and with proper metal casings are inserted in the RF conductor. One example of a high-power attenuator is a sand load comprising Aquadag-coated sand as a dissipative medium. A 50--50 mixture of coated and uncoated sand is used as a filling material for the load. A metal end plug is soldered in place to terminate the line. Loads of this type dissipate lower levels of energy and are difficult to reproduce, resulting in a high reject percentage, even for lenient maximum voltage standing wave ratio specifications. Further, the sand loads of this type suffer because they do not make use of stepped or continuous tapers at their input ends. Steps or a smooth taper facilitate impedance-matching and tend to allow a more uniform power dissipation along its length. Other large power attenuators include a coaxial-line load which makes use of a straight conical taper to the outer conductor. The load material first used with this design was polyiron made resistant to high temperatures by a ceramic binder. However, it was impossible to mold a taper lip properly and matching of such a taper into polyiron was objectionably difficult in commercial production. The search for other more suitable materials resulted in mixtures of graphite and cement.
In the microwave region, more work has been done in high-power waveguide loads than on coaxial loads since the waveguide has a greater pulse power capacity than the coaxial line, and since the skin loss in the metal walls of the line is smaller in waveguides than in coaxial lines. Sand loads made of the same composition as described above have been held in a tapered position by a Transite plate. The power limitation of loads of this type results because of the inability of the Transite material to withstand high temperatures. The waveguide tapered sand loads offer an advantage in that they have more reproducable and accurate characteristics than do the coaxial sand loads, but all of them suffer from varying impedance as the moisture absorption of the sand changes. Moreover, their construction permits shock and vibration to break and structure containing the sand, or even to change the match of the load because of a change in the compactness of the sand. Other methods of construction waveguide high-pwwer loads include using waveguide walls which are poor conductors, instead of using attenuating material which completely fills the waveguide. The dissipative material frequently used in such arrangements comprises a mixture of 35% Portland cement and 65% Dixon's No. 2 powdered flaked graphite. However, such loads require sand-graphite mixtures of precise proportions and a crumbly unworkable material results if the graphite percentage is too high, while too little graphite makes the attenuation constant too high. Further, such constructions suffer from poor heat dissipation qualities and also because if the dissipative plug fails to fit the guide well, so as to form intimate contact therewith, sparking between the walls of the guide and particles in the dielectric may ensure.
Many microwave applications include evacuated waveguide constructions. Even if the graphite content is reduced to avoid a crumbly composition, the mixture absorbs gas and moisture, rendering it unsuitable for a vacuum environment.
Coaxial loads in use today are of two types: a very bulky air-cooled arrangement or a more compact water cooled metallized glass arrangement. In the latter construction, a ceramic tubular substrate is coated with a thin film dissipative metal material. The ends of the metallized tube are painted with a conductive material to form a contact ring. Power is applied to the rings through spring loaded contact fingers which are maintained in pressure contact with the rings. Cooling water is circulated over the outer surface of the metal material in an axial direction. Locally high water flow velocities are developed in the small area where the spring finger contacts the connector ring, causing erosion of the finger and ring material and occasional separation of the finger from the ring. This induces arcing and further erosion in the contact area between the finger and the ring.
A particular problem has arisen in high vacuum, high power radio frequency cavities having several standing waves present during operation. It is frequently desired to identify and isolate unwanted out-of-phase modes, preventing their interference with desired in-phase standing wave modes. In addition to being able to withstand high continuous power dissipation and thermal cycling stresses, resistors installed in such cavities must be formed of materials which do not outgas or otherwise destroy the vacuum established in the cavity. Further difficulties are encountered in providing RF resistors for retrofit application to existing RF devices, in that space within and surrounding such equipment has been previously utilized according to the original design of the equipment.
It is therefore an object of the present invention to provide a high-power radio frequency resistor which exhibits a uniform power dissipation along its length, while incorporating high thermal efficiency heat removal means.
An additional object of the present invention is to provide a radio-frequency resistor which has accurate reproducible attenuation and load matching characteristics not affected by temperature, moisture, vibration, or thermal shock.
Also, it is an object of the present invention to provide a radio-frequency resistor of the type described above which is compact, mechanically rugged, and easy to fabricate without requiring costly equipment.
Another object of the present invention is to provide an RF resistor for a hard vacuum high temperature environment which selectively damps unwanted out-of-phase energy in a resonant RF cavity.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.